Spatial summation in human cone mechanisms from 0 degrees to 20 degrees inthe superior retina

The maximum area of complete spatial summation (i.e., Ricco's area) for hum an short-wavelength-sensitive(S-) and long-wavelength-sensitive- (L-) cone mechanisms was measured psychophysically at the fovea and at 1.5 degrees, 4 degrees, 8 degrees, and 20 degrees along the vertical meridian in the supe rior retina. Increment thresholds were measured for three observers by a te mporal two-alternative forced-choice procedure. Test stimuli ranging from - 0.36 to 4.61 log area (min(2)) were presented on concentric 12.3 degrees ad apting and auxiliary fields, which isolated either an S- or an L-cone mecha nism on the plateau of its respective threshold versus intensity function. Test flash durations were 50 and 10 ms for the S- and L-cone mechanisms, re spectively. The data indicate that, from 0 degrees to 20 degrees, Ricco's a rea increases monotonically for the L-cone mechanism, is variable for the S -cone mechanism, and is larger for the S-cone mechanism than for the L-cone mechanism for essentially all retinal locations. This pattern of results m ost likely reflects differences in ganglion cell density and changes in neu ral convergence with retinal eccentricity. (C) 2000 Optical Society of Amer ica [S0740-3232(00)01803-2].